Electrochemical reduction of aromatic compounds

ABSTRACT

A PROCESS FOR ELECTROCHEMICALLY REDUCING AROMATIC COMPOUNDS IN THE PRESENCE OF A SUBSTANTIALLY ANHYDROUS SYSTEM CONTAINING AN AMINE, AN AMMONIUM SALT OR AN INORGANIC ACID AND A CATALYST, SUCH AS BF3.

United States Patent Oflice 3,682,791 Patented Aug. 8, 1972 3,682,791 ELECTROCHEMICAL REDUCTION OF AROMATIC COMPOUNDS Joseph S. Matthews, OHara Township, Allegheny County,

Pa., asslgnor to Gulf Research & Development Company, Pittsburgh, Pa. No Drawing. Filed May 6, 1971, Ser. No. 141,017 Int. Cl. C07b 29/06; C07c /10 US. Cl. 204-59 R 19 Claims ABSTRACT OF THE DISCLOSURE A process for electrochemically reducing aromatic compounds in the presence of a substantially anhydrous system containing an amine, an ammonium salt or an inorganic acid and a catalyst, such as BF This invention relates to a process for electrochemically reducing aromatic compounds, particularly for electrochemically reducing benzene to 1,4'cyclohexadiene. The mixture being subjected to electrochemical reduction herein, by passing a direct current of electricity therethrough, is a substantially anhydrous homogeneous mixture containing an aromatic compound, an amine, an ammonium salt or an inorganic acid and a catalyst.

The aromatic compound can be naphthalene, alkyl-substituted naphthalenes, benzene or alkyl-substituted benzenes carrying one or two alkyl substituents thereon, wherein the alkyl substitutents, the same or difierent, can have from one to four carbon atoms, but preferably is methyl. Examples of such aromatic compounds are naphthalene, alpha-methylnaphthalene, beta-ethylnaphtha'lene, benzene, toluene, 0-, mor p-xylene, etc. The benzenes are converted principally to 1,4-cyclohexadienes, while naphthalene is reduced principally to 1,4-dihydronaphthalene and eventually to 1,4,5,8-tetrahydronaphthalene.

The amine, which is employed herein as a solvent, can include any amine in which the aromatic compound being reduced is soluble, and can be defined by the following structural formula:

wherein -R' can be either hydrogen or an alkyl group having from one to eight carbon atoms, preferably from one to four carbon atoms, and R" can be an alkyl group as defined by R. Specific examples of amines that can be used are methylamine, ethylamine, butylamine, diethylamine, dibutylamine, etc.

As catalyst any electron deficient compound which is soluble in the amine solvent defined above and which is not inactivated by said solvent can be used. Specific examples of catalysts that can be used are boron trifluoride, aluminum bromide, tetracyanoethylene, etc.

In addition to the above there is also present in the substantially anhydrous system an inorganic acid soluble in the reaction mixture having an ionization constant above about 1x10 preferably in the range of about 1x10"- to about 1x10- or an ammonium salt of said inorganic acid soluble in said mixture. Specific examples of inorganic acids or ammonium salts that can be used are hydrogen chloride, ammonium chloride, ammonium nitrate, ammonium bromide, etc.

The amounts of each of the components of the reaction mixture can be varied over a relatively wide range. Thus, based on the total mixture being subjected to reduction herein, the aromatic compound can be present in an amount ranging from about 0.1 to about 50, preferably from about one to about 10 mol percent; the amine from about 40 to about 90, preferably from about 60 to about mol percent; the catalyst from about 0.01 to about two, preferably from about 0.1 to about one mol percent; and the inorganic acid or ammonium salt from about one to about 10, preferably from about three to about six, mol percent.

The pressure and temperature of the mixture during the reaction are not critical. Pressure has no apparent effect on the reaction and therefore atmospheric pressure is preferred. Temperatures can be in the range of about 0 to about 50 C., but a temperature in the order of about 10 to about 30 C. is preferred. Time is not critical and is dependent upon the amount of reduction desired.

The amount of current required for electrolysis can also vary over a wide range. Thus, the amount of current supplied to the reaction system can vary from about 0.01 to about 0.3 ampere .per square centimeter of cathode surface area, preferably within the range of about 0.05 to 0.2 ampere per square centimeter of cathode surface area. The electrodes employed are carbon electrodes, preferably graphite.

The reaction is simply efiected. The reaction mixture defined above is placed in a cell or vessel containing a carbon anode and a carbon cathode and merely involves passing a direct current of electricity therethrough over a designated period of time. If desired, a divided cell can be used, for example, one containing a fritted glass disc divider. This is not preferred, however, because there is a tendency to clog up the cell divider, current efliciency is reduced, electrical resistance increases, more heat is generated, etc. The resultant mixture can then be subjected to simple distillation procedures to recover the desired reduced product.

In order to exemplify the above a number of runs were made in an undivided cell containing spectroscopic grade.

graphite rods, except Runs Nos. 12 and 14, in which a zinc or mercury cathode, respectively, was used each of which had a diameter of six millimeters and a cathode surface area immersed in the reaction of eight square centimeters. The solution during electrolysis was stirred magnetically while it was cooled externally by a water bath. During the runs the temperature was maintained at about room temperature, 15. Current was supplied by a Technipower Model L -30M power supply, wherein the voltage (0- 160 v.) or current (0-3 A.) could be regulated. Analysis was made by gas chromatography where possible and peaks, not otherwise identifiable, were identified by massgas liquid chromatography. The results obtained are set forth below in Table I and Table H.

TABLE I Current efii- Mol Molar Overeieney topercent concen- Molar Moi selectration concen- Mols percent eur- 1,4- tivity Salt of salt tration benzene rent cycloto 1,4- or or Cataoi oataben- Ampeoneflihexa- Cyeloeyclohex- M1. acid Gms. acid st zene Amps Volts hours verted eiency diene hexane adlene 50 NH Cl 1.7 0.64 BF; 0.32 0.113 0.60 60 1.80 19.7 86.4 60. 25.9 70 50 NH C1 3.2 1.28 BF; 0.64 0.226 0.50 25 2.00 3.9 30.3 21.8 8.5 72 50 NH4C1 1.7 0.64 BFa 0.32 0.113 0.60 70 2.40 21.6 66.0 48. 17.2 74 50 NH4Br 2.9 0.60 AlBr; 0.32 0.113 0. 75 30 1.50 12.8 73.5 58.2 15.3 78 50 NH4C1 1.7 0.64 31% 0.32 0.113 0.75 30 2.25 12.3 52.3 37.4 14.9 72 50 NHlGl E 1.7 0.64 0.20 0.113 0.75 32 2.25 7.2 24.0 18.3 5.7 76 50 NH Cl 67 0.25 BF3 I 0.32 0.113 0.50 85 2.00 6.5 23.0 19.1 3.9 83 50 N'H4Cl 1.13 0.50 BF j 032 0.113 0.50 50 2.00 12.1 40.6 31.2 9.4 77 50 NH C1 2.6 1.0 BFa 0.32 0.113 0. 50 20 2. 00 2.3. I 7.8 6.5 1.3 83 50 NH C1 3.2 1.28 BF; 0.32 0.113 0.60 12 1.92 3 H01 Saturated BF; 0.32 0.113 0.50 '50"- 1. 00 50 NH4C1 I 0.60 II 36 I 1.50 50 NH4C1 0: 75 50 3. 00 50 HCl- I 0.75 1.1.5 I a 50 NHSCN 0.75 3.00 I 16. EDA 50 NH4C1 0.75. 3.00 179-" EDA .50 NH4C1 I I I 0.75 48 2.25 1.4' 4.8 A 1.0 '04.

N orE'.-EDA=Ethylenediamine; TQNE =Tetraeyanoethylene, 3 I u I TABLE II v I Molar Molar "percent Current Moi conoenooncen- I8107'OYQI'B 11 efficiency to-. percent w .tratration I Mols matie current l I selec- "tion 'w of mw con- A effi-1 ..Dio1e qMonativitvto M1. Salt 'Gms. salt lyst lyst. Aromatic matte Amps. Volts hours, verted eieney fins olenns diolefin ,5o.NH4cl 1.7 0.64 BF; 0.75 45 1.5 i 1 13.4 14.3 --50 NH46l 1.7 0;64L=BF3 0.75 50 1.5 6.5 30 4 50- NH4C1 1.7 0.641BF3 0. 75 56 1.5 ,06 1226.0 4.4 50 NH C1' 1.7 0.64 BFQ' 82 0. 75 55 1.5 10.0 34.0 5.3 50 NH C1 1.7 0.64 BF: 0.32 -M ,sity- 0.082 0.75 .64 1.5 I I I ene.

M (D+2T) x 5860 A wherein M equal the mols of aromatic charge. equals percent dihydro product. according to gas chromatography and Tequals percent tetrahydro product according to gas chromatography, and A equals ampere hours used; In each of Runs Nos. 1 to 9, inclusive, benzene was'converted to a product composed mainly of 1,4fcyclohexadiene. The amount of benzene converted and the over-all current efiiciency. was dependent on the amounts of components in the reaction mixture and the amount of current applied thereto. In each of Runs Nos. 12"and 14 no conversion was obtained, since zinc and mercury'ca'thodes, respectively, were used in place of the graphite cathode of Rum Nos. 1 to 9. The use of a solvent other than an' 'amine in Runs Nos. 10, "11, 15 and 16 and the'use' of 'a salt other than anammoniumsalt'of an inorganic acidin Run 'No. 16 similarly resulted in no conversion of benzene.

Although FeCl would normally have been' expected to 3 be satisfactory herein. it is believed it did not fu'nction as a catalyst in Run No. 13 because it complexed-with the amine solvent. It-is believed that A1C13,did"l10t catalyze thereactionin Run No. 17 because it was not soluble in the reaction medium, while AlBr did serve asa c a taia e t i fore only such limitations should be imposedas are indicated in the appended claims. Iclaimz' 1 1. A process for electrochemically reducing an aromatic compound selected from the group consisting of naphtha- 713. The' process of 'claim 1 wherein the' components lene, alkylnaphthalenes, benzene andalkyl -benzenes car.- rying from one to'twoalkyl substituents-on the-ring in :a vessel containing a carbon anode and 'a carboncathode whiclr comprises passing a direct current'of electricity through'asubstantially anhydrous. homogeneous: mixture containing said aromatic compound, analiph'aticaamine 'havin g' -1-8 carbonsin' a chain, an 'electron deficient .compound soluble in the amine ascatalyst arid-an inorganic "compound selected -from -the group co'nsisting of an inorganic acid having an ionization constant above-zabout 1 "10- and the ammonium saltithereofr 4 i .2. The process-ofclaim 1 wherein saicl arom-atic compound is naphthalene.

r .3. The'process of claim lwherein saidarornat c.

pound is benzene. I i

i 4. The process of claim poundiso-xylene. 0' 5. 'The process of claim l wherein' sa fpoundis m-xylene.

6. TheIprocess of claim 1 Iwhe enediamine. j j 7. The process of claim 1 wherein said inorganic acid is HCl.

8. The process of claim 1 wherein said ammonium salt is NH CI.

9. The process of claim 1 wherein said ammonium salt is NH Br. 31 10. The process of claim 1 wherein said catalyst is 3- 11. The process of claim 1 wherein said catalyst is 2."'The wherein sai d. atom 2,00

process of claim '1 wherein said catalyst is of the re'a'ction mixture are-presentin the .followingmola'r percentages: from about 0.1 to about -50-pe'rcent aromatic icompound,'--from abnutflflto about 90.;pereent-t amine, from about0.01 to-zahout twoipercent catalysttand from about I one .to .about .10 percentaof the inorgan c acid .or

ammoniumsalt;

14. The process of. claim 1 whereinthe" components-of the rcaction'mixture: arepresent in the following molar percentages: from about one to about "10 percent-aromatic compound, from about 60 to about 80 percent amine, from about 0.1 to about one percent catalyst and from about three to about six percent of the inorganic acid or ammonium salt.

15. The process of claim 1 in which graphite electrodes are employed in the electrolysis.

16. The process of claim 1 in which the temperature of the reaction mixture is from about 0 to about 50 C.

17. The process of claim 1 in which the temperature of the reaction mixture is from about 10 to about 30 C.

18. The process of claim 1 in which the mixture being subjected to electrolysis contains benzene, ethylenediamine, BF and NH CI.

19. The process of claim 1 in which the mixture being subjected to electrolysis contains benzene, ethylene diamine, AlBr and NH Br.

References Cited UNITED STATES PATENTS F. C. EDMUNDSON, Primary Examiner US. Cl. X.R. 20473 R; 260-667 mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 7 Dat d August 8, 1972 Inventor(s) Joseph S. Matthews It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 57, "l60-3OM" should read "160-3M" Column 3, Table I, Run No. 9 under "Grams", "2.6"

should be "2.7".

Signed and sealed this 23rd day of January 1973.

(SEAL) Attest- EDWARD M.PLETCHER,JR. ROBERT GOTTSCHALK Atte'stin-g Officer Commissioner of Patents 

